1. Technical Field of the Invention
The present invention relates to a device for imparting a predetermined motion to an object by using a cam, and provides a cam driving device adapted to reduce variations in the force of contact between the cam and a cam follower which variations are caused by the motion of a driven section, so as to achieve high-speed high-precision motion.
2. Description of the Prior Art
Conventional cam driving devices, particularly a cam device for machining (video tape recording) cylinders which demand precision, will now be described.
FIG. 1 shows a typical VTR cylinder section, wherein a tape 1 is entrained around the outer peripheral surfaces of an upper rotary cylinder 5 and a lower fixed cylinder 3 in such a manner that the edge of the tape 1 extends along a tape guide 4 (hereinafter referred to as a lead) provided on the outer peripheral surface of the lower fixed cylinder 3, said tape 1 traveling at a fixed speed. A magnetic head 6 is attached to the upper rotary cylinder 5 to move diagonally of the tape 1 during rotation of said upper rotary cylinder 5 and serves to record and reproduce magnetic signals on the tape or to reproduce magnetic signals previously recorded on the tape.
If, therefore, the tape guide 4 comprising the outer peripheral surfaces of the upper and lower cylinders 5 and 3 and the lead 4 provided on the lower cylinder 3 is not correctly produced and assembled, this would result in unstable contact between the tape 1 and the magnetic head 6, causing unevenness in reproduced images in the VTR.
For example, lack of true circularity in the cylinders 3 and 5 varies the state of contact between the magnetic head 6 and the tape 1, while lack of linearity in the lead 4 wrinkles the tape 1, each causing trouble in reproducing images.
The lead 4 on the lower cylinder 3 is generally produced on a lathe, as shown in FIG. 2. More particularly, a cutting tool 9 fixed to the tool rest 8 is reciprocated in the direction on arrows in synchronism with the rotation of the lathe main spindle 7 to produce a predetermined shape. Thus, changes in the position of the tool rest 8 in reciprocating motion must agree with the shape of the lead 4.
Conventionally, the method most widely practiced for giving the tool rest 8 a motion agreeing with a predetermined lead shape has, as shown in FIG. 3, been to attach to the main spindle 7 of the lathe an end cam 10 having a curve similar to the shape of the lead 4 drive the tool rest 8 as a function of the curve of the end cam 10. The numeral 12 denotes a guide for the reciprocating linear motion of the tool rest 8, and 13 denotes a spring interposed between the guide 12 and a reciprocating rod 14 for urging a cam follower 11 against the end cam 10.
In addition, though not shown in FIG. 3, the reciprocating rod 14 is internally provided with a mechanism for feeding the cutting tool 9 during lead machining. Phenomena which take place during the machining of the lead 4 by using the end cam 10 described above are as follows.
(1) With changes in the position of the reciprocating rod 14, the amount of deflection of the spring 13 changes and so does the force of contact between the surface of end cam 10 and the cam follower 11.
(2) The inertia effect of the reciprocating rod 14 due to its reciprocating motion varies the force of contact between the surface of end cam 10 and the cam follower 11, and in the case of high-speed motion, the cam follower 11 jumps off the cam surface.
(3) Vibration of the entire machining system takes place with the reciprocating motion of the reciprocating rod 14.
These phenomena produce serious influences on the machining of the lead 4 which requires a high accuracy of the order of microns. More particularly, variations in the force of contact between the cam surface and the cam follower 11, i.e., the force on the end cam 10, not to speak of the phenomenon of the cam follower 11 jumping off the cam surface, aggravate the accuracy of rotation of the main spindle 7, producing a decisive influence on the true circularity of the cylinder 3. Further, vibration of the machining system must be avoided as much as possible.
With the conventional cam driving device, which has the drawbacks described above, it has been very difficult to achieve high speed machining while maintaining high accuracy.